SOME in Brooklyn want to know: Was Mayor Bloomberg issuing a threat to the unruly firefighters of Engine Co. 204? Or a promise?

In a sneaky move that took firemen and Cobble Hill moms by surprise, the mayor showed up this weekend for a private sit-down with some of the guys who work in the doomed firehouse. On Palm Sunday morning. At the crack of 8.

With almost no warning, a posse of four, black Suburban SUVs came rumbling up to the 150-year-old firehouse. Surprise! Into the outer-borough light emerged the mayor, flanked by Chief of Department Frank Cruthers and Chief of Operations Salvatore Cassano.

The sight proved puzzling to the early churchgoers and homecoming club-hoppers who were sentient at that hour. The mayor would barely have had time for a nap after he finished singing “The Impossible Dream” at the Inner Circle show in Midtown, in the wee hours of that very morning.

But here he was, in the city’s outer circle, reprising his song in a private performance designed for an audience of on-duty firefighters.